1. Technical Field
The present invention relates to an electrode for polymer electrolyte fuel cells, and in particular, relates to a technology in which a catalytic layer functions efficiently.
2. Background Art
A polymer electrolyte fuel cell is formed by laminating separators at both sides of a tabular membrane electrode assembly (MEA). The membrane electrode assembly is typically a laminated body having a polymer electrolyte membrane placed between a cathode side catalytic layer and an anode side catalytic layer, and having a gas-diffusion layer laminated at the outside of each catalytic layer. This catalytic layer is formed by a method in which a catalyst paste is coated on an electrolyte membrane and these are hot pressed, or by a method in which a catalyst paste is coated on a carbon paper or FRP sheet to form an electrode sheet, which is hot pressed with an electrolyte membrane. The catalyst paste is composed of a catalytic material such as Pt, an electron conducting material such as carbon particles, and an ion conducting material such as polymer electrolyte. A bonding agent and a water-repelling agent such as PTFE (polytetrafluoroethylene) or PVDF (poly(vinylidene fluoride)) are added if necessary.
In such a fuel cell, a fuel gas (such as hydrogen) supplied through a separator plate arranged at the cathode side and an oxidizing gas (such as air) supplied through a separator plate arranged at the anode side are diffused through each gas-diffusion layer, and each gas reaches the catalytic layers. In the catalytic layer of the anode side, the fuel gas reacts and generates protons and electrons, and in the catalytic layer of the cathode side, protons, electrons, and oxidizing gas react and generate water, thereby generating electricity.
Therefore, it is necessary to supply fuel gas and oxidizing gas uniformly to the entirety of the catalytic layer to improve generation efficiency. The inventors further researched researching the voids in the catalytic layer functioning as gas channels, that is to say, the most suitable structure for the voids in the catalytic layer.
Japanese Unexamined Patent Application Publication No. 92293/97 discloses that voids having diameters of from 0.04 to 1.0 μm function as gas channels, and gas is adequately diffused if the specific volume of the voids is 0.04 cm3/g or more. However, the void volume described above is controlled by the amount of the ion conducting material or the conditions of hot pressing. Therefore, the amount of the ion conducting material or the temperature and the pressure in the hot pressing process must be reduced to increase the voids (decrease the ion conducting channels), thereby causing deterioration in the adhesion between the membrane and the electrode. It is difficult to obtain a void density of 0.1 cm3/g or more in this method.
In Japanese Unexamined Patent Application Publication No. 203840/94, an electrode, in which the porosity of the catalytic layer is 65 to 90 vol %, is disclosed. However, voids of this electrode are also controlled by conditions of hot pressing.
Furthermore, although the catalytic layer which is made by the process described above has voids to a certain extent before it is hot pressed, the voids are crushed and reduced after the hot pressing process, so that it is impossible to increase the voids to be more than that before the hot pressing process.
In order to solve this problem, Japanese Unexamined Patent Applications Publication No. 203852/94, No. 236762/94, No. 176310/95, No. 180879/96, No. 199138/97, No. 3929/98, No. 189005/98 and No. 189012/98 propose methods in which void forming agents such as zinc powder, silica sol, ammonium hydrocarbonate, camphor, lithium carbonate, or the like, are added to a catalyst paste, and are removed after the hot pressing process, thereby increasing the voids. However, these methods require a process to remove the void forming agent after the hot pressing process, and this causes a complicated production process.
Japanese Unexamined Patent Application Publication No. 223503/97 proposes a method in which voids are increased by adding a solvent having a high boiling point into a catalyst paste and evaporating the solvent in a sintering process. However, this method also requires an additional sintering process and also results in a complicated manufacturing process.
Furthermore, Japanese Unexamined Patent Application Publication No. 329452/99 proposes a method in which an ink (A) including an ion conducting material and an ink (B) including a solvent which does not exhibit ionomer lytic potential, such as methyl dodecate, are mixed to produce an ink (C), and after forming the catalytic layer by this ink (C), evaporating the solvent such as methyl dodecate, and thus the voids are increased. However, this method requires two kinds of inks and the atmosphere of the evaporating process must be strictly controlled.
Japanese Unexamined Patent Application Publication No. 223233/98 proposes a method in which carbon fibers are added to a catalyst paste so that the paste does not fill the voids of a porous substrate (gas-diffusion layer). However, this electrode is composed of a catalytic layer which is formed on the inside surface of a carbon paper, and this is different from the structure of the catalytic layer of the present invention which does not include carbon paper.
It is advantageous to reduce unnecessary material as much as possible to improve the generation efficiency of a polymer electrolyte fuel cell. PTFE and PVDF mentioned above are unnecessary from the viewpoint of power generation, and it is undesirable to add these materials. However, if these bonding agents are not added, the catalytic layer cannot maintain its structure by itself Furthermore, because the catalytic layer is a thin membrane and its bonding strength is low, cracks may occur and exfoliation of the catalytic layer and the substrate may occur during the manufacturing process even if the catalytic layer could maintain its structure.
Though much research has been performed thus far to improve generation efficiency per volume of catalytic layer, actually, a satisfying result has not yet been obtained. Therefore, an object of the present invention is to provide an electrode for polymer electrolyte fuel cells comprising the catalytic layer having a void structure exhibiting high generation efficiency without a complicated manufacturing process.